In video and still pictures processing, down-sampling and up-sampling techniques are widely used either to adapt the content of a picture to the spatial resolution of the display or to pre-process a sequence of pictures before the encoding step in order to achieve a given bit-rate. The down-sampling and up-sampling filters conventionally used are linear low-pass filters.
The combination of the known down-sampling and up-sampling linear filters leads to blurry images because high frequencies are attenuated by said filters. Additional techniques can be introduced in order to enhance or to create high frequencies so as to improve edges. But image enhancement techniques that are conventionally used after the up-sampling filter, namely they correspond to post-processing techniques, are often complex.
An example of such a post-processing technique called peaking is described in the paper entitled “A generic 2D Sharpness Enhancement Algorithm for Luminance Signals”, by E. G. T. Jaspers and P. H. N. de With, Sixth International Conference on Image Processing and Its Applications, 1997, Volume: 1, pp. 269-273, 14-17 Jul. 1997. Roughly speaking, peaking consists in enhancing high frequencies by adding to an up-sampled image the result of a high pass filtering of said up-sampled image multiplied by a weighting coefficient.
Furthermore, such processing techniques operate on the whole up-sampled image, and thus yield to many computations.